Many modern work machines operate in relatively rugged environments where traction can be difficult to maintain. Work machines such as motor graders must often operate on loose soil, gravel, etc., imparting a tendency for one or more wheels of the work machine to slip as the machine is driven across a work surface. When the work machine is turning, however, it is desirable to allow the wheels to rotate at different speeds, or “differentiate.” To this end, most modern work machines include a differential that allows the wheels to rotate at different speeds, in a familiar manner. When low traction conditions are encountered, however, differentiation of the wheels can have the undesired effect of providing motive power or “rim pull” predominantly or solely to a spinning wheel on a given axle, while the other wheel sits idle.
In the past, wheel slip problems have been addressed in a variety of ways. One method of addressing wheel slip problems is to selectively apply a brake to slow a slipping wheel such that rim pull may be returned to the opposite wheel. In one strategy, when slip of one wheel of a particular axle is detected, the operator or an electronic controller can selectively actuate a brake on the subject wheel to slow it down, allowing the spinning wheel to regain some traction and returning power to the opposite wheel. While selective braking systems work well in certain environments, they are not without limitations. One such limitation relates to the heat generated by friction of the brake on components of the wheel, ultimately transmitting heat into the axle. Because of this phenomenon, there are limitations as to how much a wheel can be selectively braked until the temperature exceeds that which the work machine can tolerate. In addition, selective braking obviously results in extra wear and tear on the brakes of the work machine.
Another approach to wheel slip relates to selectively locking the differential of one or more axles of a work machine. As discussed above, during normal operation, the differential allows wheels coupled with the axles to rotate at different speeds, in turn allowing the wheels to rotate independently as the work machine navigates a turn. Both operator controlled and computer controlled systems are known whereby a differential clutch is engaged via an actuator to couple rotation of the wheels of an axle together. Rotationally coupling the wheels together can reduce wheel slip and resultant loss of traction in a well-known manner. A problem inherent to operator controlled differential locks, as well as certain computer controlled systems, is the potential for overuse and under use of the differential locks. In particular, operators are often occupied with a plethora of concerns and controls while operating the work machine, and have been known to give less attention to the differential locks. As a result, the differentials are often locked in situations where they would be better unlocked, and vice versa.
For example, when a work machine with a locked differential is entering a turn, it will typically be desirable to unlock the differential. If the differential remains locked through a turn, the outside wheel and the inside wheel components, including the tires, can each experience significant stress, as can components of the powertrain. In some cases gear teeth on certain of the powertrain gears can actually break. Even worse, where the work machine takes the turn with sufficient speed, an operator's failure to unlock a differential can even cause the work machine to roll over. Despite the undue wear and tear on the machine, many operators opt to keep the differential locked inappropriately rather than risk losing traction and reducing operating efficiency.
On the other hand, where a work machine with an unlocked differential encounters poor underfoot conditions, one of the wheels of each driven axle may spin significantly before the operator decides to activate the differential lock, also resulting in wear on the wheels, tires and powertrain components. Despite wear and tear on the machine resulting from overuse and under use of the differential locks, operators are obviously unable to focus their attention solely on controlling the differential. While it would of course be desirable to strike a perfect balance between wear and tear on the one hand, and operating efficiency on the other, even the most highly skilled operators are not equipped to monitor and control all the aspects of work machine operation that would be necessary to achieve such a goal.
Various electronic control strategies have been developed which automate at least some of the control over the differential locking state in a work machine, allowing the operator to better attend to other machine functions and attempting to reduce overuse and under use of the differential locks. One such strategy is known from Japanese Patent Application Publication No. 2003-237619 to Kenji (hereinafter “Kenji”). Kenji is directed to a differential lock switching device for a work vehicle that switches the locking state of a differential based on a detected articulation angle or a detected wheel steering angle. The development is described primarily in the context of a motor grader having a hydraulic differential lock control valve, apparently for use with a combination pneumatic and hydraulic differential lock. While Kenji provides an approach that will facilitate electronic control over differential locking and unlocking, Kenji's strategy fails to account for many of the machine stress conditions set forth above. Thus, many of the shortcomings inherent in an operator controlled strategy would still inhere in Kenji's approach. Kenji's apparent use of simple wheel steering and articulation thresholds to trigger locking or unlocking also provides only a sometimes appropriate locking and/or unlocking the differential, and therefore sacrifices operating efficiency. Operating efficiency would appear to be less than optimal in particular because Kenji does not appear to vary the thresholds to account for different operating conditions.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.